This invention relates to a control and propulsion mechanism for use in aircraft in general and helicopters in particular.
All aircraft require some means to control the direction of the aircraft and to propel the aircraft along its flight path. Helicopters have either one or two main lifting rotors. In tandem rotor helicopters, those helicopters having two lifting rotors, both the propulsive force and those forces required for directional or yaw control are provided by the main rotors. In single rotor helicopters the propulsive force is provided by the main lifting rotor while yaw control has generally been provided by a second and smaller rotor located at the rear or tail of the fuselage.
An inherent aspect of controlling the yaw of the single rotor helicopter is the counteraction of torque generated in driving the main rotor of the helicopter. This torque tends to rotate the entire aircraft in a direction opposite to the rotation of the main lifting rotor. The torque is generated by the resistance of the air to the driving of the rotor. The force required to counteract the torque is relatively large compared to the amount of force required to vary the attitude of the aircraft about its yaw axis. In the event of a power failure, the main rotor "autorotates" or rotates freely permitting the helicopter to "glide" to a safe landing. During autorotation there is a substantial, if not complete, elimination of the torque which tends to rotate the aircraft.
Other mechanisms have been used to control the directional heading of single rotor helicopters and to counteract torque. One manner in which this has been accomplished has been to use two lifting rotors mounted on a common shaft wherein the rotors rotate in opposite directions. The torque generated by these two rotors counteract one another. By changing the torque of one rotor relative to the other directional control is achieved. Another prior art manner in which directional control is accomplished has been to mount jets at the tail of the rotorcraft.
It is generally recognized that helicopters are inherently limited in the maximum velocity which they may achieve. This limitation results from the fact that the main rotor, in addition to supplying lift for the aircraft, must also maintain the aircraft in a stable position about its roll axis. In order to accomplish this the left half of the rotor must generate an amount of lift equal to that generated by the right half of the rotor. As helicopter velocity increases, this becomes increasingly difficult to achieve.
As a helicopter rotor moves forward through air, the velocity of the air passing over the individual rotor blades varies depending upon whether the rotor blade is advancing or retreating with respect to the freestream air. When the blade advances, the velocity of the air passing over the blade is equal to the linear velocity due to rotation of the blade plus the velocity of the aircraft through the air. When the blade retreats, the velocity of the air passing over the blade is decreased by the velocity of the aircraft. This velocity difference results in the advancing blade generating more lift than the retreating blade. In order to compensate for this phenomena the angle of attack of the rotor blades is decreased while it is advancing so as to generate less lift and increased while it is retreating so as to increase lift. In this manner the two halfs of the rotor disc generate equal amounts of lift thus maintaining the aircraft in a stable position about its roll axis. The aircraft velocity can increase to the point where this equilibrium cannot be maintained while the aircraft is in level flight. The maximum velocity of the aircraft can be increased beyond this point if lift is sacrificed thus resulting in loss of altitude of the aircraft.
The maximum velocity of the aircraft can also be increased if additional propulsion means are provided. Since the rotor provides both lift, propulsion and control about the roll axis, an auxiliary propulsion device permits that energy, or a portion thereof, which is used to propel the aircraft forward to be used to maintain the aircraft in a stable position about its roll axis.
Another matter in which the maximum velocity of a helicopter has been increased has been to supply the aircraft with small wings which provide some lift at higher velocities. The rotor is then required to generate less lift at the higher velocities enabling more of the energy to be used to propel the aircraft and control roll.
Structures have been proposed in the prior art which control yaw and provide thrust augmentation through the use of jets of air exiting from nozzles in the rearward portion of a helicopter fuselage. They have proven unsatisfactory in that sufficient yaw control was not provided. Another problem with the prior art structures has been the existence of a time delay between actuation of the pilot controlled members and the initiation of the desired control. Additionally, some of these structures have required so much energy, primarily due to losses in changing the direction of air flow, that they were not practical. A particularly accute problem with such structures has been the unsatisfactory, if not complete absence, of yaw control during autorotation and at low speeds. In some cases during autorotation yaw control, if present, has been available in only one direction.